A principle is something, usually a rule or norm, that is part of the basis for something else. For example, the ethics of someone may be seen as a set of principles that the individual obeys. These principles form the basis for their ethics.
Principles may also be introduced as pedagogy: laying down basics in a topic, in order later to proceed to more detailed developments.
Identifying or defining a rule as a principle says that, for the purpose at hand, the principle will not be questioned or further derived. This is a convenient way of reducing the complexity of an argumentation.
See also: principled approach.
wiktionary:principle